1. Field of the Invention
This invention relates to a multi-piece solid golf ball of a four layer structure comprising a core consisting of an inner sphere and an enclosure layer surrounding the inner sphere and a cover surrounding the core and consisting of outer and inner layers. More particularly it relates to such a multi-piece solid golf ball which is improved in spin performance upon approach shots with a sand wedge etc. and presents a soft hitting feel upon approach shots and putting and at the same time, travels an increased distance and gives a pleasant soft hitting feel upon full shots with a driver independent of whether the head speed is high or low.
2. Prior Art
Two-piece solid golf balls comprising a rubber based core and a cover of ionomer resin or the like around the core offer superior flight performance and durability although they have the drawback of a hard hitting feel. To eliminate this drawback, various soft type two-piece solid golf balls were developed. To obtain such soft type two-piece solid golf balls, soft cores are generally used. Softening the core invites not only a lowering of restitution which leads to poor flight performance, but also a substantial loss of durability. Then the flight performance and durability characteristic of two-piece solid golf balls are lost to such an extent that some soft type two-piece solid golf balls are practically unacceptable.
To overcome these problems, a number of three-piece solid golf balls were proposed. Exemplary golf balls attempted heretofore include (1) a three-piece solid golf ball comprising a core consisting of inner and outer layers and a cover surrounding the core wherein the core consists of a soft, relatively small inner layer (outer diameter: 24 to 29 mm, hardness: Shore D 15 to 30) and a hard outer layer (outer diameter: 36 to 41 mm, hardness: Shore D 55 to 65) surrounding the inner layer whereby a long carry is ensured as well as a hitting feel and controllability close to wound golf balls (Japanese Patent Publication (JP-B) No. 55077/1992 and Japanese Patent Application Kokai (JP-A) No. 80377/1989); (2) a three-piece solid golf ball comprising a center core, an intermediate layer, and a cover wherein a soft intermediate layer is formed around a soft center core and the thickness and specific gravity of the center core, intermediate layer, and cover are selected in specific ranges whereby the feeling is improved at no sacrifice of flight performance and durability (JP-A 24084/1995); and (3) a three-piece solid golf ball comprising a center core, an intermediate layer, and a cover wherein a relatively hard intermediate layer is formed between a relatively soft core and a relatively soft cover whereby the feeling and controllability are improved at no sacrifice of flight performance and durability (JP-A 24085/1995).
However, these golf balls suffer from various problems. The ball (1), in which the cover is not particularly limited, provides insufficient restitution and fails to travel a long distance when a soft member is used as the cover. When a hard member is used as the cover, the cover and the underlying core outer layer are hard so that upon approach shots belonging to the low deformation region, the hitting feel becomes hard. The ball (2) gives a good feel upon driver shots because the core and intermediate layer are soft, but gives a hard feel on sand wedge shots and gains a spin rate insufficient to control the ball because the cover is hard. The ball (3), in which the core that mostly affects feel and restitution is soft, provides insufficient restitution and fails to travel a long distance as long as the hitting feel is fully soft. As long as the restitution is sufficient, the core is relatively hard so that the hitting feel is not fully soft. Additionally, the intermediate layer is also hard so that low-head speed players cannot provide the ball with sufficient deformation to travel a long distance. There still remains room for further improvement.
An object of the present invention, which has been made under the aforementioned circumstances, is to provide a multi-piece solid golf ball comprising a core consisting of an inner sphere and an enclosure layer surrounding the inner sphere and a cover surrounding the core and consisting of outer and inner layers, which is improved in spin performance upon approach shots with a sand wedge etc. and presents a soft hitting feel upon approach shots and putting and at the same time, travels an increased distance and gives a pleasant soft hitting feel upon full shots with a driver independent of whether the head speed is high or low.
Making extensive investigations in order to attain the above object, the inventors have found that when a multi-piece solid golf ball comprising a core consisting of an inner sphere and an enclosure layer surrounding the inner sphere and a cover surrounding the core and consisting of outer and inner layers is formed as a four layer structure comprising a core consisting of a soft inner sphere and a relatively hard enclosure layer and a cover surrounding the core and consisting of a soft outer layer and a hard inner layer, (1) the cover outer layer formed soft is effective for improving spin performance upon approach shots with a sand wedge and makes soft the hitting feel upon approach shots and putting, (2) the use of a hard resilient resin as the cover inner layer is effective for maintaining satisfactory flight performance, and (3) the inner sphere formed relatively soft and the relatively hard, resilient enclosure layer surrounding the inner sphere are effective for presenting a very soft hitting feel while maintaining high restitution upon full shots with a driver independent of whether the head speed is high or low.
Continuing further extensive investigations based on the above findings (1) to (3), the inventors have found that the problems associated with prior art three-piece solid golf balls can be effectively solved when a multi-piece solid golf ball comprising a core consisting of an inner sphere and an enclosure layer surrounding the inner sphere and a cover surrounding the core and consisting of an outer layer and an inner layer is constructed such that the outer layer of the cover has a hardness of 40 to 60 in Shore D, the inner layer of the cover has a hardness of 55 to 70 in Shore D, the surface hardness of the enclosure layer is higher in Shore D than the surface hardness of the inner sphere, the inner sphere has a hardness expressed by a distortion of 3.0 to 8.0 mm under an applied load of 100 kg, and the ratio of the hardness A of the inner sphere to the hardness B of the ball, both expressed by the distortion under an applied load of 100 kg, is in the range: 1.1xe2x89xa6A/Bxe2x89xa63.5. More specifically, upon full shots with a driver which is in the large deformation region of the ball, especially by low-head speed players, the resulting three-piece solid golf ball is given a sufficient deformation. That is, the ball receives an appropriate spin rate to travel a drastically increased distance and gives a very pleasant soft hitting feel while maintaining high restitution independent of whether the head speed is high or low. In addition, upon approach shots with a sand wedge belonging to the small ball deformation region, the ball is susceptible to spin and easy to control. Furthermore, upon approach shots and putting, the ball gives a soft pleasant feel. The present invention is predicated on these findings.
Accordingly, the present invention provides:
(1) a multi-piece solid golf ball comprising a core consisting of an inner sphere and an enclosure layer surrounding the inner sphere and a cover surrounding the core and consisting of an outer layer and an inner layer, characterized in that the outer layer of said cover has a hardness of 40 to 60 in Shore D, the inner layer of said cover has a hardness of 55 to 70 in Shore D, said enclosure layer has a surface hardness higher than the surface hardness of said inner sphere in Shore D, said inner sphere has a hardness expressed by a distortion of 3.0 to 8.0 mm under an applied load of 100 kg, and the ratio of the hardness A of said inner sphere to the hardness B of the ball, both expressed by a distortion under an applied load of 100 kg, is in the range: 1.1xe2x89xa6A/Bxe2x89xa63.5;
(2) a multi-piece solid golf ball of (1) wherein said inner sphere is formed mainly of a rubber base material and has a diameter of 20 to 37 mm, and said core has a diameter of 32 to 41 mm;
(3) a multi-piece solid golf ball of (1) or (2) wherein the cover outer layer has a gage of 0.3 to 3.0 mm, the cover inner layer has a gage of 0.5 to 3.0 mm, and the difference in hardness between the outer layer and the inner layer is at least 5 in Shore D; and
(4) a multi-piece solid golf ball of (1), (2) or (3) wherein the hardness of the cover inner layer is higher than the hardness of the cover outer layer.